#include<iostream>
using namespace std;
int w, h, n, r;
bool st[110][110];
int main()
{
	cin >> w >> h >> n >> r;
	memset(st, 0, sizeof(st));
	int ans = 0;
	while (n--) {
		int x, y;
		cin >> x >> y;
		for (int i = x - r; i <= x + r; i++) {
			for (int j = y - r; j <= y + r; j++) {
				if (i >= 0 && i <= w && j >= 0 && j <= h) {
					int dis = (i - x) * (i - x) + (j - y) * (j - y);
					if (dis <= r * r) {
						st[i][j] = 1;
					}
				}
			}
		}
	}
	for (int i = 0; i <= h; i++){
		for (int j = 0; j <= w; j++) {
			ans += st[i][j];
		}
	}
	cout << ans << endl;
	return 0;
}